1. FIELD OF THE INVENTION
The present invention relates to a multi-stage cold accumulation type refrigerator and a cooling device utilizing the same.
2. DISCUSSION OF THE INVENTION
FIG. 29 shows a conventional three-stage GM (Gifford-McMahon) refrigerator as a multi-stage cold accumulation type refrigerator as disclosed in Advances in Cryogenic Engineering Vol, 15, p428, 1969, for example. The refrigerator includes a third cold accumulator 1 having a cold accumulating member formed of lead balls, a second cold accumulator 2 having a cold accumulating member formed of lead balls, a first cold accumulator 3 having a cold accumulating member formed of copper wire net, a third displacer 4, a second displacer 5, a first displacer 6, a third seal 7 for preventing leakage of a helium gas 16 from an outer periphery of the third displacer 4, a second seal 8 for preventing leakage of the helium gas 16 from an outer periphery of the second displacer 5, a first seal 9 for preventing leakage of the helium gas 16 from an outer periphery of the first displacer 6, a three-stepped cylinder 10 formed from a honing pipe, a suction valve 11 for inducing the helium gas 16 compressed by a helium compressor 13, an exhaust valve 12 for exhausting the helium gas 16, a driving motor 15, a driving mechanism 14 for converting rotation of the driving motor 15 into a linear motion and operating the suction valve 11 and the exhaust valve 12 in synchronism with the linear motion, third, second and first expansion chambers 17, 18 and 19 for expanding the helium gas 16, a third thermal stage 20 for transmitting cold generated in the third expansion chamber 17 to a body to be cooled (not shown), a second thermal stage 21 for transmitting cold generated in the second expansion chamber 18 to the body, and a first thermal stage 22 for transmitting cold generated in the first expansion chamber 19 to the body.
The operation of the above refrigerator will now be described. FIG. 30 is a P-V diagram in the expansion chambers 17 to 19, wherein an axis of ordinate represents a pressure in the expansion chambers 17 to 19, and an axis of abscissa represents a volume of the expansion chambers 17 to 19. Under the condition as shown by (1), the displacers 4 to 6 are disposed as their uppermost positions, and the suction valve 11 is open, while the exhaust valve 12 is closed. Accordingly, the pressure in the expansion chambers 17 to 19 is a high pressure PH. When the condition is shifted from (1) to (2), the displacers 4 to 6 are lowered, and the helium gas 16 having a high pressure is induced through the cold accumulators 1 to 3 into the expansion chambers 17 to 19. During this operation, the valves 11 and 12 remain still. The helium gas 16 is cooled to predetermined temperatures by the cold accumulators 1 to 3. Under the condition at (2), the volume of each expansion chamber is maximum, and the suction valve 11 is closed, while the exhaust valve 12 is opened. At this time, the pressure of the helium gas 16 in each expansion chamber is reduced to generate cold, and the condition is shifted to (3). When the condition is shifted from (3) to (4), the displacers 4 to 6 are raised, and the helium gas 16 having a low pressure is exhausted. At this time, the helium gas 16 cools the cold accumulators 1 to 3, and the temperature of the helium gas 16 is increased. Then, the helium gas 16 is returned to the helium compressor 13. Under the condition at (4), the volume of each expansion chamber is minimum, and the exhaust valve 12 is closed, while the suction valve 11 is opened. As a result, the pressure in each expansion chamber is increased to restore the condition shown by (1).
In the multi-stage cold accumulation type refrigerator as mentioned above, the efficiency of the third cold accumulator is rapidly reduced, and temperature of 6.5 K or less can not be obtained because a specific heat of lead forming the cold accumulating member of the third cold accumulator is smaller temperature of 10 K or less, while a specific heat of helium gas is large.
Further, a generated refrigeration quantity becomes smaller than an indicated refrigeration quantity at a temperature of 4 K owing to a change in physical property of helium. Accordingly, there occurs a problem of heat generation due to sliding resistance of the seal.
Further, as the specific heat of the third heat stage becomes small at temperature of about 4 K, temperature oscillation in a refrigeration cycle is increased to cause a reduction in efficiency.
If the cold accumulating member in the conventional multi-stage cold accumulation type refrigerator is formed of an alloy or compound containing a rare earth metal (which alloy or compound will be hereinafter referred to as a rare earth substance), fine powder of the cold accumulating member is generated by vibration during operation, and is deposited to the seal portions, causing a reduction in sealing effect and an increase in friction between each displacer and the cylinder.